Episode 46: Obstetric Pharmacology and Fetal Assessment with Mike Hofkamp

In this episode, episode 46, I welcome back Dr. Mike Hofkamp to discuss obstetric pharmacology and fetal assessment.

6 Replies to “Episode 46: Obstetric Pharmacology and Fetal Assessment with Mike Hofkamp”

  1. Thank you for this very informative podcast. I am interested to know more about diffusionak hypoxia in the neonatal post Maternal N2O during delivery. The use of supplental oxygen in the neonate is certainly not standard practice here in Australia. N2O is used widely here from both labor and GA Caesarean section. Could you please direct me to some literature to support this.
    Kinds regards
    Hanna Burton
    (Anaesthetic trainee)
    Gold Coast, Aust

    1. Hi Hanna,

      Great question. I asked Dr. Hofkamp and this is his response. Thanks for listening!

      The trainee from Australia is correct that most of the time the nitrous oxide levels in the infant are negligible, but it is recommended to administer supplemental oxygen nonetheless.

      Mankowitz E, Brock-Utne JG, Downing JW. Nitrous oxide elimination by the newborn. Anaesthesia. 1981;36(11):1014-6

      Abstract:
      The elimination of nitrous oxide by ten infants whose mothers had received 50% nitrous oxide in oxygen and enflurane 0.6-1% during general anaesthesia for Caesarean section was studied. The concentration of nitrous oxide detected in end-expired gas ranged from 1 to 4 vol% (mean 1.9 vol%). These levels are too low to produce significant diffusion hypoxia in vigorous neonates. However, a minority of infants may be adversely affected and it is recommended that oxygen-enriched air be administered to infants whose mothers have received nitrous oxide.

  2. I always love coming back to these ACCRAC podcasts after having not listened for a while…I always find something new! All shows are so well researched and so well done they have definitely enhanced my experience as an anesthesia learner. I was re-listening to this podcast last week after going back for my 3rd rotation on OB and found our policy was to continue magnesium infusions for neuroprotection in pre-eclamptic patients during C-sections. My question is in regards to magnesiums effect on depolarizing NMBs. In this episode Dr. Hofkamp correctly states that magnesium will potentiate a non-depolarizing block. A quick pubmed search didn’t return any relevant studies on magnesium’s effect on a depolarizing block. A 2011 review on magnesium by Dr. Susanne Herroeder in Anethesiology stated,
    “MgSO4does not interfere with onset and duration of succinylcholine-induced neuromuscular block but seems to prevent associated muscle fasciculations and may attenuate potential succinylcholine-induced increases of serum potassium.51,52 ”
    but the references discussed the fasciculations and potassium, not the block. chapter 34 in Big Miller 8th edition states,
    “the interaction between magnesium and succinylcholine is controversial. However, more recent results suggest that magnesium antagonizes the block produced by succinylcholine. 258”
    referencing an 1994 study by Tsai and colleagues in the Brittish Journal of Anaesthesia on cats.

    Morgan and Mikhail 6th edition, on the other hand, in table 11-3 lists magnesium as potentiating a depolarizing block, and an open anesthesia article and podcast focusing on keyword review point to a 1983 study by McLarnon and colleagues in the Journal of Neuroscience showing that magnesium decreased post-synaptic motor endplate currents, making it seem plausible that mg could potentiate a depolarizing block.

    I have only been involved in a handful of c-sections on pre-eclamptic that got put to sleep, so I don’t feel like I have a lot of real world experience to compare with. Do we know what effect magnesium has on a depolarizing block in a pregnant or even non-pregnant patient?

    Thanks!

    Leif Ericksen
    CA-2, Stony Brook University

    1. Hi Leif,

      You’ve summarized the evidence incredibly well and I don’t have much to add. I don’t think we know for sure. If it has a clinical effect it isn’t all that significant. If I hear differently from any of our OB folks, or from Dr. Hofkamp, I’ll let you know. Thanks for the well researched comment!

      Best,
      Jed

      1. Here is Dr. Hofkamp’s response:

        Thank you for the question. I admit that I haven’t thought much about the effect of magnesium on the depolarizing block of succinylcholine. I agree that there isn’t much clinical data out there. One study I found was from the 1950’s and was actually very helpful. del Castillo & Engbaek did a study entitled “The Nature of the Neuromuscular Block Produced by Magnesium” and found that magnesium blocks neuromuscular transmission by 1) decreasing the amount of acetylcholine produced by a motor transmission and 2) decreasing the response of the motor end plate to acetylcholine. Significantly, the authors excluded the possibility that magnesium had depolarizing activity by itself, suggesting that phase II block with magnesium is unlikely. Additionally, the authors found that calcium antagonizes the neuromuscular blocking effects of magnesium.

        When I provide general anesthesia for cesarean sections, I almost always administer succinylcholine in a rapid sequence induction and rarely have to use a non-depolarizing agent for additional neuromuscular blockade. Amazingly, the obstetricians seem to get along fine with muscle relaxation exclusively from the volatile/nitrous anesthetic. I remember one case in particular earlier in my career where I gave a non-depolarizing neuromuscular blocking agent to a patient on a magnesium infusion and she failed extubation and required postoperative ventilation in the surgical intensive care unit. Since then, I have been reticent to give a non-depolarizing neuromuscular blocking agent unless it is absolutely unavoidable. With the addition of sugammadex to our practice, I would reverse a non-depolarizing block with sugammadex as a first line agent on a patient who was on a magnesium infusion.

        Take home message: there is no evidence that magnesium has depolarizing action on the neuromuscular junction and is thus unlikely to potentiate a phase II block. If anything, magnesium could antagonize the action of succinylcholine.

        del Castillo J, Engbaek L. The Nature of the Neuromuscular Block Produced by Magnesium. J Physiol 1954;120(4):54P

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